Hubble finds that nearest quasar is powered by binary black hole

The Space Telescope Science Institute Press Release

This artistic illustration is of a binary black hole found in the centre of the nearest quasar to Earth, Markarian 231. The binary black holes are predicted to spiral together and collide within a few hundred thousand years. Mrk 231 is located 600 million light-years away. Image credits: NASA, ESA, and G. Bacon (STScI).Astronomers using NASA’s Hubble Space Telescope have found that Markarian 231 (Mrk 231), the nearest galaxy to Earth that hosts a quasar, is powered by two central black holes furiously whirling about each other.

The finding suggests that quasars — the brilliant cores of active galaxies — may commonly host two central supermassive black holes that fall into orbit about one another as a result of the merger between two galaxies. Like a pair of whirling skaters, the black-hole duo generates tremendous amounts of energy that makes the core of the host galaxy outshine the glow of the galaxy’s population of billions of stars, which scientists then identify as quasars.

Scientists looked at Hubble archival observations of ultraviolet radiation emitted from the centre of Mrk 231 to discover what they describe as “extreme and surprising properties.”

If only one black hole were present in the centre of the quasar, the whole accretion disc made of surrounding hot gas would glow in ultraviolet rays. Instead, the ultraviolet glow of the dusty disc abruptly drops off towards the centre. This provides observational evidence that the disc has a big donut hole encircling the central black hole. The best explanation for the observational data, based on dynamical models, is that the centre of the disc is carved out by the action of two black holes orbiting each other. The second, smaller black hole orbits in the inner edge of the accretion disc, and has its own mini-disc with an ultraviolet glow.Markarian 231 (UGC 8058) is a Type-1 Seyfert galaxy in the constellation Ursa Major that was discovered in 1969 as part of a search of galaxies with strong ultraviolet radiation. Results from the first spectrum showed clear signs of the presence of a powerful central quasar. Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).“We are extremely excited about this finding because it not only shows the existence of a close binary black hole in Mrk 231, but also paves a new way to systematically search binary black holes via the nature of their ultraviolet light emission,” said Youjun Lu of the National Astronomical Observatories of China, Chinese Academy of Sciences.

“The structure of our universe, such as those giant galaxies and clusters of galaxies, grows by merging smaller systems into larger ones, and binary black holes are natural consequences of these mergers of galaxies,” added co-investigator Xinyu Dai of the University of Oklahoma.

Proposed model of Mrk 231. Two supermassive black holes, each with their own mini-disc, orbit each other in the centre of a circumbinary disc. The secondary black hole has cleared gap in the circumbinary disc as a result of its orbit around the primary black hole. Image credit: Chang-Shuo Yan et al. 2015.The central black hole is estimated to be 150 million times the mass of our Sun, and the companion weighs in at 4 million solar masses. The dynamic duo completes an orbit around each other every 1.2 years.

The lower-mass black hole is the remnant of a smaller galaxy that merged with Mrk 231. Evidence of a recent merger comes from the host galaxy’s asymmetry, and the long tidal tails of young blue stars.

The result of the merger has been to make Mrk 231 an energetic starburst galaxy with a star-formation rate 100 times greater than that of our Milky Way galaxy. The infalling gas fuels the black hole “engine,” triggering outflows and gas turbulence that incites a firestorm of star birth.

The binary black holes are predicted to spiral together and collide within a few hundred thousand years. Mrk 231 is located 600 million light-years away.

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